Adamantyl carboxylic and sulfonic acid catalyzed paraffin isomerization

ABSTRACT

A process is described for paraffin isomerization under strong acid conditions in which an adamantyl carboxylic acid or sulfonic acid is used to substantially increase the reaction rate of the isomerization.

BACKGROUND OF THE INVENTION

This invention relates to a catalytic process for paraffin isomerizationproducing branched paraffins under strong acid catalyzed conditions inthe presence of adamantyl carboxylic or sulfonic acids, as hydridetransfer catalysts.

Alkylation or isomerization of paraffins under strong acid conditionsare well-known processes for producing a wide variety of usefulhydrocarbon materials and particularly, gasoline additives. For example,2,2,4-trimethylpentane is a common blending agent used for gasolineoctane improvement which can be produced by alkylating isobutylene withisobutane in sulfuric acid or liquid HF. An example of such anacid-catalyzed reaction process is described in U.S. Pat. No. 3,231,633.

Hydrocarbon conversion processes employing novel Lewis acid systems aredisclosed in U.S. Pat. No. 4,229,611 and U.S. Pat. No. 4,162,233, bothassigned to Exxon Research and Engineering Company.

U.S. Pat. No. 3,671,598 describes a process for isomerizing saturatedcyclic hydrocarbons under strong acid conditions in the presence of anadamantyl hydrocarbon. However, no suggestion is made that otherspecifically substituted adamantyl derivatives, particularly those withcarboxy or sulfoxy substituents, might be more effective in increasingthe rate of isomerization of paraffins to branched isomers.

U.S. Pat. Nos. 4,357,481; 4,357,484; 4,357,482; and 4,357,483 to GeorgeM. Kramer (issued Nov. 2, 1982, and assigned to Exxon Research andEngineering Company) disclose the use of adamantane hydrocarbons inparaffin-olefin alkylation and non-cyclic paraffin isomerization, andthe use of aminoalkyladamantanes in paraffin-olefin alkylation andnon-cyclic paraffin isomerization, respectively, in which rates ofreaction are substantially increased as compared to the absence of thespecifically disclosed adamantane. However, none of the patents discloseor suggest the use of carboxy- or sulfoxy-containing adamantanes as rateenhancing agents in alkylation or isomerization process.

New methods for producing such branched paraffinic hydrocarbons areconstantly being searched for in an effort to improve isomerizationefficiency. More active catalysts would enable these rearrangements tobe conducted at lower temperatures where thermodynamic equilibria aremore favorable to branched structures, an important factor in butane,pentane and hexane isomerization.

SUMMARY OF THE INVENTION

We have found that the presence of an adamantyl carboxylic or sulfonicacid in a strong acid system containing an isomerizable paraffinichydrocarbon significantly increases the rate of isomerization of saidhydrocarbon, presumably because the paraffin becomes involved in fasterintermolecular hydride transfer reactions. Since inter-molecular hydridetransfer is generally the rate-determining step in paraffinisomerization, (see "Industrial Laboratory Alkylation", edited by LyleF. Albright and Arthur R. Goldsby, ACS Symposium Series 55, PublishedWashington, D.C., 1977, Chapter One, "Alkylation Studies" by G. M.Kramer) the presence of the adamantyl derivative acid will serve tosignificantly increase the reaction rate of the isomerization process.In the production of octane-increasing agents, this should lead to theutilization of smaller and more efficient reactors, which enhances theeconomics of the process.

More specifically, by this invention, there is provided an isomerizationprocess comprising the step of contacting a C₄ -C₆ paraffinichydrocarbon with a strong acid system in the presence of an adamantylcarboxylic acid, adamantyl sulfonic acid, or mixture thereof, saidadamantyl compound containing at least one unsubstituted bridgeheadposition, and said process being conducted at a temperature of about-100° C. to 150° C., thereby producing a branched isomer of saidparaffinic hydrocarbon.

DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

The reason that an adamantane carboxylic acid or adamantane sulfonicacid serves to increase the rate of intermolecular hydride transferduring branched paraffin isomerization, is not totally understood. Onetheory that we do not wish to be bound by is that reversible hydridetransfer from the bridgehead position of the adamantyl group to acarbonium ion in solution is enhanced due to lack of steric repulsionsin the transition state involving the adamantyl group, as compared tohydride transfer involving a paraffinic hydrocarbon and the samecarbonium ion.

In the process, C₄ -C₆ paraffinic hydrocarbons are isomerized. As iswell-known, the extent of the rearrangement and the possibility ofchanging the degree of branching of the paraffin, as distinct from thepossibility of inducing an alkyl shift, depends primarily on the acidsystem. The adamantane derivative catalyzes the process appropriate tothe acid employed. Examples of operable paraffins include n-butane,isobutane, isopentane, n-pentane, 2-methylpentane, 3-methylpentane,n-hexane, mixtures thereof, and the like. Preferred paraffins in theprocess are 2- and 3-methylpentane, n-hexane, n-pentane and n-butane, orrefinery streams containing mixtures of these components which are notat their equilibrium concentrations.

Both normal and branched paraffins can be used in the subjectisomerization process, under very strong acid conditions, e.g., AlBr₃-CH₂ Br₂ solutions. However, with slightly weaker acid systems, such asH₂ SO₄ and HF, isomerizations are limited to the rearrangement ofparaffins containing a tertiary carbon atom. Normal paraffins liken-butane do not generally undergo the isomerization process in theseweaker acid systems.

The product paraffins in the process are C₄ -C₆ branched paraffinichydrocarbons. Representative examples include isobutane, isopentane,2-methylpentane, 3-methylpentane, 2,3-dimethylbutane,2,2-dimethylbutane, and the like. The preferred product paraffinichydrocarbons in the process are the most highly branched isomerspossibly derivable from each of the C₄, C₅ and C₆ product streams. Theproduct paraffins are useful as gasoline blending agents for octaneimprovement and/or hydrocarbon solvents. Higher molecular weightparaffins may also be isomerized. However, it is generally known thatsuch reactions are often accompanied by extensive side reactions.Specific requirements for the selective isomerization of such reagentsare beyond the scope of this patent application.

The phrase "a strong acid system", as used herein, refers to the acidsystem capable of assisting in generating carbonium ions in the processand includes an "acid component" and a solvent, or one material that canfunction in both capacities, such as concentrated sulfuric acid,preferably being of initial acid strength of about 95 to 99 weightpercent, or liquid HF. The acid system can be solid/liquid, liquid, orgaseous. Preferably the acid system is a liquid and particularlypreferred is concentrated sulfuric acid as the acid system.

The strong acid components in the acid system are conventional protic,aprotic, or Lewis acids and include AlBr₃, AlCl₃, GaCl₃, TaF₅, SbF₅,AsF₅, BF₃, HF, HCl, HBr, H₂ SO₄, HSO₃ F, CF₃ SO₃ H, and the like andmixtures thereof. A preferred acid component in the process, when aimedat preparing most highly branched products, is AlCl₃, AlBr₃, GaCl₃, orTaF₅ used with a solvent as described hereinbelow. If a rapid butlimited rearrangement is desired, H₂ SO₄ or HF would be the preferredacids. An example of the former is the isomerization of n-hexane todimethylbutanes and an example of the latter is the isomerization of2-methylpentane to 3-methylpentane. Also, HCl and HBr are preferably notused alone, but are used in combination with other Lewis acids, e.g.,AlCl₃ or AlBr₃.

Also a component of the "acid system", may be a solvent for the acidcomponent as where the strong acid component is a solid material, e.g.,AlBr₃. For Lewis acids, halogenated paraffins and aromatics cangenerally be used; representative examples include CH₃ Br, CH₂ Br₂, CH₂Cl₂, 1,2-dichloroethane, 1,2,3-trichlorobenzene,1,2,3,4-tetrachlorobenzene, pentafluorobenzene, HF, H₂ SO₄, CF₃ SO₃ H,HSO₃ F and the like, and mixtures thereof.

The molar concentration of acid component in the solvent, if they aredifferent materials, is generally between 0.1 and 8.0 M, and preferably0.5 to 4.0 M (moles/liter) based on solvent volume.

The volume ratio of the acid system to the paraffinic hydrocarbon to beisomerized is generally about 5:1 to 1:5, and preferably about 3:1 to1:3 parts by volume. However, larger and smaller ratios can beeffectively employed.

Isomerization reactions described herein are normally carried out intwo-phase systems, i.e., an acid phase containing the adamantylderivative acid and a hydrocarbon phase. The system can also be apartially miscible one where, for example, a paraffin, AlBr₃ and1,2,3,4-tetrachlorobenzene are employed.

The adamantane acid compound useful in the process contains at least onecarboxy or sulfoxy group, preferably being an alkylcarboxy oralkylsulfoxy group and at least one unsubstituted adamantyl bridgeheadposition, is preferably surface active, and can be prepared byconventional methods in the art. By the term "surface active", is meantthat the adamantane acid compound depresses the surface tension of theacid system, and promotes formation of an emulsion between the acidphase and hydrocarbon phase, when used at low concentration, typicallyin the range of 10⁻⁶ to 10⁻¹ moles/liter based on the liquid acid layer.

The adamantyl carboxylic acid or sulfonic acid is preferably of theformula: ##STR1## where n=0-16, preferably 1-12, most preferably 4-8,A=COOH or SO₃ H, and wherein the adamantane ring and the alkyl chain canbe further modified and substituted with groups which are inert underthe process conditions and include C₁ -C₄ alkyl groups, NO₂ and CF₃ orC_(n) F_(2n+1) (where n=1-10) replacements for the remaining adamantylbridgehead protons provided that at least one adamantyl bridgeheadhydrogen remains to promote intermolecular hydride transfer.

Further included are adamantane compounds in which a total of 2 or 3 ofthe 4 bridgehead protons of the adamantane ring are replaced by a(CH₂)_(n) --COOH or (CH₂)_(n) --SO₃ H chain, n varying from 0 to 16.

The alkyl chains can also contain non-reactive branches, e.g.,--(CH₂)_(n) --C--(CH₃)₂ --(CH₂)_(m) --A, where n=(0-10), m=(0-10) and Ais --CO₂ H or --SO₃ H, where the total carbon chain is no more than 16carbon atoms in length. The neopentyl structure existing in the aboveillustrated chain is non-reactive in all but the strongest acids andthus, can be used in concentrated H₂ SO₄ or HF solutions. Expresslyexcluded is a single methyl group substitution, or its equivalent, whichcan form reactive tertiary carbonium ions in the process.

Representative examples include 16-(1'-adamantyl)hexadecanoic acid,12-(1'-adamantyl)dodecanoic acid, 4-(1'-adamantyl)butanoic acid,3-(1'-adamantyl)propanoic acid, 2-(1'-adamantyl)ethanoic acid,1'-adamantyl carboxylic acid, 10-(1'-adamantyl)decanoic acid,8-(1-adamantyl)octanoic acid, 6-(1'-adamantyl)hexanoic acid,6-(2'-adamantyl)hexanoic acid, 5-(1'-adamantyl)-2-methylpentanoic acid,5-(1'-adamantyl)pentanoic acid, 6-(1'-adamantyl)hexylsulfonic acid,5-(1'-adamantyl)pentylsulfonic acid, 4-(1'-adamantyl)butylsulfonic acid,4-(2'-adamantyl)butylsulfonic acid, 12-(1'-adamantyl)dodecanoic acid,and the like. A preferred catalyst compound is 6-(1'-adamantyl)hexanoicacid. It should also be noted that readily solvolyzed derivatives ofthese acids and their equivalents such as their esters, anhydrides,acylhalides and amides, which generate the corresponding free acidthrough solvolysis under "protic acid" reaction conditions, cangenerally be used in place of the parent adamantane compounds and areincluded within the scope of the claimed subject process.

The molar concentration of adamantyl compound in the acid solutionvaries from about 10⁻⁶ to 10⁻¹ moles/liter, and preferably about 10⁻⁴ to10⁻² moles/liter. However, larger and smaller ratios can also be usedeffectively.

Temperatures in the process are conducted in the range of about -100° to150° C. and preferably about -50° to 100° C., depending primarily on thetemperature required to obtain a liquid-phase catalyst.

The process is normally carried out at atmospheric pressure but may alsobe conducted at higher pressures up to about 20 atmospheres, thepressure depending primarily on the partial pressure of isobutane in thereaction mixture.

Yields of isomeric hydrocarbons in the process are only limited by thethermodynamic equilibrium at the process temperature, and it is withinthe scope of this invention to separate product isomers and recycle thestarting material and less desirable product materials for furtherconversion to the more desirable isomers.

A particularly preferred embodiment of the process is where n-butane isisomerized to isobutane, n-pentane is isomerized to isopentane, andn-hexane is isomerized to a mixture of methylpentanes anddimethylbutanes.

Apparatus for carrying out the subject process is conventional, eitherin a laboratory, pilot plant, or full industrial scale, and the processcan be conducted in a batch-type operation or in a continuous-typeoperation in liquid/liquid or liquid/gas systems. The adamantyl compoundmay also be used in solid/liquid or solid/gas systems, wherein its polarfunctionality is adsorbed onto or bound by a highly acidic solid acid. Apreferred process is a liquid/liquid system conducted in a continuousmanner.

Generally, the process is conducted by contacting a liquid mixture ofparaffin with the adamantyl compound in the acid system describedherein. If the acid system is, for example, concentrated H₂ SO₄, theprocess is conducted in an emulsion of the two-phase system, the acidphase usually being the continuous phase, although this is not essentialto the process. The entire system is preferably at reaction temperatureat time of mixing, during which the entire system is vigorously mixed,stirred and agitated to insure good contact between the acid andhydrocarbon phases. The reaction is allowed to progress until a desiredor substantial quantity of formed product is obtained. This can bemonitored by analytical methods such as gas chromatography and massspectrometry. After the desired paraffinic product has been formed, thephases can be separated and the hydrocarbon phase treated by extractionor fractional distillation, and the like, to separate out and collectthe desired product.

It is to be understood that obvious modifications and variations on theabove-described procedure and subject process, not specificallydescribed herein, are deemed to be encompassed within the general scopeand spirit of this application.

The following example is illustrative of the best mode of carrying outthe invention, as contemplated by me, and should not be construed asbeing limitations on the scope or spirit of the instant invention.

EXAMPLE 1

This example illustrates the effect of surface active adamantylcarboxylic acids in accelerating intermolecular hydride transfer at asulfuric acid/hydrocarbon interface resulting in faster isomerization ofa branched paraffin, i.e., 3-methylpentane to 2-methylpentane. Listed inthe Table are the relative isomerization rates of 3-methylpentane,obtained under well-stirred two-phase conditions, using equal volumes ofthe 3-methylpentane and 96% sulfuric acid, which contained the listedadamantyl carboxylic acids. For comparison, the isomerization rate withno adamantyl additive is also listed.

The no additive run was conducted by mixing 100 ml. of conc. H₂ SO₄,(96%) with 100 ml. of 3-methylpentane in a 500 ml., 2-neck flask at roomtemperature and atmospheric pressure. The two-phase system was stirredvigorously and samples of the upper hydrocarbon phase were withdrawnperiodically and analyzed by gas chromatography for the extent ofisomerization. The reaction was then individually repeated with 0.002 M.solutions of the three listed adamantylalkyl carboxylic acids insulfuric acid. The relative isomerization rates in the systems weremeasured. As seen in the data, the net isomerization rate of3-methylpentane to 2-methylpentane more than tripled when a 0.002 M.solution of any one of the adamantylalkyl carboxylic acids used, ascompared to the no additive control. Also seen is the fact thatincreasing the carbon chain length of the alkanoic acid had asignificant effect on increasing the isomerization rate.

                  TABLE                                                           ______________________________________                                        Comparison of Surface Active                                                  Hydride Transfer Catalysts in H.sub.2 SO.sub.4                                               3 Methylpentane                                                Catalyst.sup.(a)                                                                             Isomerization, Rel. Rate.sup.(b)                               ______________________________________                                        None           1                                                              1'-Ad-(CH.sub.2).sub.3 COOH                                                                  3.5                                                            1'-Ad-(CH.sub.2).sub.4 COOH                                                                  3.9                                                            1'-Ad-(CH.sub.2).sub.5 COOH                                                                  7.1                                                            ______________________________________                                         .sup.(a) Adamantyl catalysts used in 0.002 M. concentrations, in              concentrated H.sub.2 SO.sub.4, 96%, at 23 ± 1° C.                   .sup.(b) Relative isomerization rates based on the control run in the         absence of adamantyl catalyst.                                           

What is claimed is:
 1. An isomerization process comprising the step ofcontacting a C₄ -C₆ paraffinic hydrocarbon with a strong acid system inthe presence of an adamantyl carboxylic acid, adamantyl sulfonic acid,or mixture thereof, said adamantyl containing at least one unsubstitutedbridgehead position, and said process being conducted at a temperatureof about -100° C. to 150° C., thereby producing a branched isomer ofsaid paraffinic hydrocarbon.
 2. The process of claim 1 wherein saidparaffinic hydrocarbon is selected from 3-methylpentane,2-methylpentane, n-hexane, n-pentane, n-butane, isomers thereof, andmixtures thereof.
 3. The process of claim 1 wherein said acid systemcontains an acid component selected from AlCl₃, AlBr₃, GaCl₃, TaF₅,SbF₅, AsF₅, BF₃, HF, HBr, HCl, concentrated H₂ SO₄, HSO₃ F, CF₃ SO₃ H,and mixtures thereof.
 4. The process of claim 3 wherein said acid systemfurther contains a solvent selected from CH₃ Br, CH₂ Br₂, CH₂ Cl₂,1,2-dichloroethane, 1,2,3-trichlorobenzene, 1,2,3,4-tetrachlorobenzene,HF, concentrated H₂ SO₄, HSO₃ F, CF₃ SO₃ H, and mixtures thereof.
 5. Theprocess of claim 3 wherein said acid component is concentrated H₂ SO₄.6. The process of claim 1 wherein said adamantyl carboxylic acid oradamantyl sulfonic acid is of the formula: ##STR2## where n=0-16, A=COOHor SO₃ H, and wherein the adamantyl ring and alkylene chain can besubstituted with substituents which are inert or unreactive under theprocess conditions.
 7. The process of claim 6 wherein A in said formulais COOH.
 8. The process of claim 7 wherein said adamantane carboxylicacid is 4-(1'adamantyl)butanoic acid, 5-(1'-adamantyl)pentanoic acid, or6-(1'-adamantyl)hexanoic acid.
 9. The process of claim 8 wherein saidadamantylalkyl carboxylic acid is 6-(1'-adamantyl)hexanoic acid.
 10. Theprocess of claim 1 wherein said adamantyl acid is present in aconcentration of about 10⁻⁶ to 10⁻⁶ moles per liter based on said strongacid system.
 11. The process of claim 1 wherein said temperature is inthe range of about -50° to 100° C.
 12. The process of claim 1 beingconducted in a continuous manner.
 13. The process of claim 1 whereinsaid branched paraffin is 3-methylpentane, said product is2-methylpentane and said acid system is H₂ SO₄.
 14. The process of claim1 wherein said strong acid system contains AlCl₃, AlBr₃, GaCl₃, or TaF₅.15. The process of claim 14 wherein n-butane is isomerized to isobutane.16. The process of claim 14 wherein n-pentane is isomerized toisopentane.
 17. The process of claim 13 wherein n-hexane is isomerizedto a mixture of dimethylbutanes and methylpentanes.
 18. The process ofclaim 14 wherein said paraffin is a mixture of C₄, C₅ or C₆ isomers,wherein at least one carbon fraction is not at thermodynamicequilibrium.
 19. An isomerization process comprising the step ofcontacting 3-methylpentane with concentrated sulfuric acid containing6-(1'-adamantyl)hexanoic acid, present in about 0.002 molarconcentration, at ambient temperature and pressure thereby producing2-methylpentane.